The present invention relates to a vertical deflection circuit of a cathode-ray tube for use in televisions and display monitors.
In an in-line color picture tube for use in televisions and display monitors, the central axis of the vertical deflection magnetic field of the deflection yoke sometimes may deviate from the central axis of the color picture tube, or the vertical deflection magnetic field sometimes may make rotative deviation with respect to the central axis the color picture tube. In the above-mentioned deviated states, the electron beam is not completely converged in the upper and lower parts of the image plane of the color picture tube, so that mis-convergence occurs in these parts.
Japanese Laid-open Patent Application No. Hei 8-102270 discloses to provide two auxiliary coils on the electron gun side of the deflection yoke in order to independently correct the mis-convergence in the upper and lower parts of the image plane. According to this prior art, a current corrected by a convergence correction circuit including a diode bridge circuit is fed through these auxiliary coils.
FIG. 7 is a circuit diagram showing the convergence correction circuit 20 of the vertical deflection yoke according to the prior art. FIG. 8 is a view showing the impedance characteristic of the vertical deflection yoke.
The circuit diagram of FIG. 7 shows the vertical deflection yoke having a vertical deflection coil 60 attached to a color picture tube 70, the convergence correction circuit 20 constituting a part of a vertical deflection circuit, vertical coma aberration correction coils 209a, 209b, 209c and 209d, and auxiliary coils 210a and 210b provided on the above-mentioned electron gun side. The convergence correction circuit 20 has a first serially-coupled pair of resistors 201 and 202. A second serially-coupled group of a diode 203, a variable resistor 205 and a diode 204 of the same direction as that of the diode 203 is connected in parallel with the first serially-coupled pair. Further, a third serially-coupled group of a diode 206 of the opposite direction to that of the diode 203, a variable resistor 208 and a diode 207 of the same direction as that of the diode 206 is connected in parallel with the first serially-connected pair.
In the convergence correction circuit 20, as shown in FIG. 8, the impedance of the vertical deflection yoke steeply increases in the vicinity of the center of the vertical deflection range, that is, in the vicinity of where the vertical deflection current is zero, based on the voltage-current characteristics of the diodes 203, 204, 206 and 207. For this reason, the vertical deflection voltage supplied to the vertical deflection yoke cannot follow the change in impedance of the vertical deflection yoke. Therefore, the deflection speed of the electron beam in the vertical direction decreases in the vicinity of the center of the image plane of the color picture tube. Consequently, a density of horizontal scanning lines increases in the central part of the image plane, so that a horizontal belt-shaped high-luminance part appears in the vicinity of the vertical-direction center of the image plane. It is known that the belt-shaped high-luminance part appears when a correction circuit utilizing the voltage-current characteristics of diodes is added to the vertical deflection circuit.
An object of the present invention is to prevent the vertical deflection speed from decreasing in the vicinity of the center of the image plane of the color picture tube, and reduce the luminance of the horizontal belt-shaped high-luminance part when the impedance of the vertical deflection yoke steeply changes.
A vertical deflection circuit of the present invention comprises: a vertical deflection voltage output circuit for supplying a vertical deflection voltage to a vertical deflection yoke of a cathode-ray tube; and a vertical linearity correction circuit for correcting the vertical deflection voltage supplied to the vertical deflection yoke so as to follow the change in input impedance of the vertical deflection yoke.
According to this configuration, the vertical deflection voltage corrected so as to follow the change in impedance of the vertical deflection yoke is supplied to the vertical deflection yoke. Consequently, the vertical deflection speed in the vicinity of the vertical-direction center of the image plane of the color picture tube is prevented from decreasing, so that the luminance of the horizontal belt-shape high-luminance part can-be reduced to a level that causes no problem in practical use.
In a vertical deflection circuit according to another aspect of the present invention, the above-mentioned vertical linearity correction circuit has negative-feedback circuits for correcting the output waveform of a vertical output amplifier in the vertical deflection voltage output circuit and feeding it back to the invert input of the vertical-output amplifier.
By the negative-feedback circuits, a corrected vertical deflection voltage that follows the change in impedance of the vertical deflection yoke is supplied to the vertical deflection yoke. Consequently, the vertical deflection speed in the vicinity of the vertical-direction center of the image plane of the color picture tube is prevented from decreasing, so that the luminance of the horizontal belt-shape high-luminance part can be reduced to a level that causes no problem in practical use.
In a vertical deflection circuit according to another aspect, the above-mentioned vertical linearity correction circuit has at least one diode and at least one resistor.
According to this configuration, by selecting the resistance value of the resistor, the vertical deflection yoke is supplied with a vertical deflection voltage appropriately corrected so as to follow the change in impedance of the vertical deflection yoke according to the voltage-current characteristic of the diode. Consequently, the vertical deflection speed in the vicinity of the vertical-direction center of the image plane of the color picture tube is prevented from decreasing, so that the luminance of the horizontal belt-shape high-luminance part can be reduced to a level that causes no problem in practical use.
In a vertical deflection circuit according to yet another aspect, the vertical linearity correction circuit has at least one first diode and at least one second diode connected in parallel with the first diode in the opposite direction.
The vertical deflection yoke is supplied with a vertical deflection voltage corrected so as to follow the change in impedance of the vertical deflection yoke according to the voltage-current characteristics of the two diodes operating on positive and negative vertical deflection voltages, respectively. Consequently, the vertical deflection speed in the vicinity of the vertical-direction center of the image plane of the color picture tube is prevented from decreasing. Therefore, the luminance of the horizontal belt-shape high-luminance part can be reduced to a level that causes no problem in practical use.
In a vertical deflection circuit according to still another aspect, the diodes constituting the vertical linearity correction circuit has substantially the same voltage-current characteristic as the diode of the convergence correction circuit provided in the vertical deflection yoke.
The diodes of the vertical linearity correction circuit can be switched between on and off in the same timing as the diode provided in the convergence correction circuit. Therefore a vertical deflection voltage corrected so as to follow in the same timing as it follows before the impedance characteristic of the deflection vertical yoke changes can be supplied to the vertical deflection yoke. Consequently, the vertical deflection speed in the vicinity of the vertical-direction center of the image plane of the color picture tube is prevented from decreasing, so that the luminance of the horizontal belt-shape high-luminance part can be reduced to a level that causes no problem in practical use.
In a vertical deflection circuit according to still another aspect, the resistor constituting the vertical linearity correction circuit has at least one first resistor connected in series with the diodes and at least one second resistor connected in parallel with the diodes.
The vertical deflection voltage supplied to the deflection yoke can be adjusted by the composite resistance value of the first resistor and the composite resistance value of the second resistor. Therefore a vertical deflection voltage appropriately corrected so as to follow the change in impedance of the vertical deflection yoke can be supplied to the vertical deflection yoke. Consequently, the vertical deflection speed in the vicinity of the vertical-direction center of the image plane of the color picture tube is prevented from decreasing, so that the luminance of the horizontal belt-shape high-luminance part can be reduced to a level that causes no problem in practical use.
In a vertical deflection circuit according to still another aspect, the resistor constituting the vertical linearity correction circuit is a variable resistor.
The vertical deflection voltage supplied to the vertical deflection yoke can be adjusted to an optimal value by adjusting the resistance value of the variable resistor. Therefore, a vertical deflection voltage appropriately corrected so as to follow the change in impedance of the vertical deflection yoke can be supplied to the vertical deflection yoke. Consequently, the vertical deflection speed in the vicinity of the vertical-direction center of the image plane of the color picture tube is prevented from decreasing, so that the luminance of the horizontal belt-shape high-luminance part can be reduced to a level that causes no problem in practical use.
In a vertical deflection circuit according to still another aspect, the diodes constituting the vertical linearity correction circuit are attached so that their temperatures are the same as that of the vertical output amplifier.
According to this configuration, the temperature conditions of the vertical output amplifier and the diodes of the vertical linearity correction unit can be made the same. Consequently, the vertical deflection circuit is not affected by change in on resistance value of the diodes due to the temperature characteristics of the diodes and the vertical deflection speed in the vicinity of the vertical-direction center of the image plane of the color picture tube is prevented from decreasing, so that the luminance of the horizontal belt-shape high-luminance part can be reduced to a level that causes no problem in practical use.